Multi-layer metal coated diamond abrasives with an electrolessly deposited metal layer

ABSTRACT

Multi-layer coated diamond abrasive particles having improved wear performance in abrasive tools are provided, wherein the coating comprises a single homogenous, carbide forming metal primary layer, preferably of chromium, and at least one non-carbide forming secondary layer applied by electroless deposition, preferably comprised of nickel/phosphorus or cobalt/phosphorus.

BACKGROUND OF THE INVENTION

This invention relates to diamond abrasive particles having amulti-layer metal coating and processes for producing the same. Thesecoated diamond abrasives find particular use in sintered metal bondedtools where the multi-layer coating aids retention of the diamondabrasives within such tools and also aids tool wear resistance.

It is well known in the art that metal coatings can improve theretention of diamond abrasive particles in the matrices of abrasivetools such as those used to saw stone and concrete. Metal coated diamondabrasives are commercially available with nickel coatings typicallyapplied by electroless deposition. While such abrasives provide goodperformance, improvements are desired to reduce the premature loss ofdiamond abrasive particles and reduce the wear of abrasive tools.

Nickel coatings applied by electroless deposition chemically bound tothe diamond surface. Metals which adhere to diamond surfaces morestrongly are well known and include molybdenum, titanium and chromium,which are carbide formers and are typically chemically vapor-depositedor sputtered onto diamond surfaces. Examples of such coatings andprocesses for depositing them are disclosed in U.S. Pat. No. 3,465,916;EP-A-79/300,337.7; U.S. Reissue No. 34,133; and U.S. Pat. No. 4,063,907.Although these coatings bond more strongly to diamond surfaces thannickel coatings, these coatings are usually oxidized and can be brittle,depending on the carbide formed.

Carbide forming metal layers have been used as part of multi-layercoatings on diamond particles to aid retention within a tool matrix.U.S. Pat. No. 3,924,031 discloses a multi-layer coating for diamondparticles wherein the first layer comprises an alloy with a base metalof copper, nickel or iron and a carbide-forming metal such as titanium,chromium or vanadium. This alloy layer may be over coated with anotherlayer such as nickel by electroless or electrolytic deposition. Thealloys comprise at most 30 wt % of the carbide forming metal and, toform the carbide, the coating is heated at high temperatures afterdeposition by vacuum evaporation or sputtering.

U.S. Pat. No. 4,378,975 describes the use of chromium as a first coatingon pelletized diamond particles are in turn used to form abrasivebodies. A sintered copper/nickel alloy forms the outer wear-resistantcoating on these pelletized particles. It is unknown whether thechromium layer forms a carbide, although green pellets are sintered attemperatures of 900° C. in forming the pelletized particles.

U.S. Pat. No. 5,024,680 describes the use of a chromium, titanium orzirconium carbide-forming layer as part of a multi-layer coating ondiamond particles to aid retention within a matrix. Two carbide-forminglayers are applied; one thin base layer and a thick oxidation-resistantsecondary layer. A Third non-carbide-forming layer applied byelectroless techniques is optional. The base carbide layer of chromium,zirconium or titanium is applied by metal vapor deposition, preferablyfollowed by heating of the coated particle to form the carbide. Chemicalvapor deposition of this layer is said to provide no advantage. Thesecondary carbide-forming metal layer of tungsten or tantalum can beapplied by CVD followed by heating of the layer to provide adequatecarburization.

These procedures for applying multi-layer coatings are complex in thateither metal alloys are applied as one of the layers, or three distinctlayers are used. In addition, these procedures provide increased bondingstrength between the diamond particles and the tool matrix throughcarburization of the metal coating, during which the diamond particlesare exposed to high temperatures. High temperatures can causedegradation of the diamond crystal, which is detrimental to theperformance of the cutting tool. Chen et al. (U.S. Pat. No. 5,024,680)recognizes this problem but provides no solution other than to avoidexcess carburization.

It is desirable to apply multi-layer coatings to diamond by a simplermethod which will aid its retention within the matrix of an abrasivetool without degrading the diamond particle and improve tool wearresistance. It is also desirable to provide multi-layer coatings todiamond which will enhance the wear performance of an abrasive tool.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide metalcoated diamond abrasive particles with improved wear performance withinsintered metal bonded abrasive tools comprising only two metal layers.

Another object of this invention is to provide a process for makingmetal coated diamond abrasive particles having a strongly adherent,multi-layer metal coating that aids tool wear performance which does notrequire exposure of these particles to thermal cycles which cause theirdegradation.

It is a further object of the present invention to provide abrasivetools with improved wear resistance which comprise multi-layer metalcoated diamond abrasive particles having a strongly bonded chromium baselayer and a compatible secondary layer (applied by electrolessdeposition).

Upon further study of the specification and appended claims, furtherobjects and advantages of this invention will become apparent to thoseskilled in the art.

These and other objects are achieved by a process least two metallayers. A carbide-forming layer provides the primary layer and ischemically bonded to the surfaces of the diamond abrasive particles.These carbide-forming layers are comprised of tungsten, titanium,tantalum, zirconium, molybdenum, hafnium, chromium, vanadium, silicon,niobium, or a carbide, boride, or nitride thereof. The outer metal layercomprises nickel, cobalt, iron, or alloys thereof, applied byelectroless deposition. The secondary coating typically ranges from10-50 wt %, preferably 20-35 wt %, of the uncoated diamond abrasiveparticles. The abrasive tools provided by this invention comprisemulti-layer metal coated diamond abrasive particles bonded within asintered metal matrix. Conventional methods for bonding the coatedparticles within a matrix to form tools can be utilized.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Diamond abrasive particles utilized in this invention are of the sizeconventionally used in sintered metal bonded tools such as, for example,those of 20/80 U.S. mesh size. The size of the particles can vary widelywithin the range of 1/1500 μm, preferably 150-1000 μm, and mostpreferably 200-600 μm. Conventionally sized diamond abrasive particlesare sufficiently large so as to provide a cutting profile for the toolsdesired and not be excessively diluted by the metal coatings to beapplied.

The diamond abrasive particles used in this invention can be natural orsynthetic but are typically obtained by conversion of graphite underhigh pressure and high temperature (HP/HT), either with or without acatalyst. Preferably, the diamonds are of a size within the range offrom 20 to 80 U.S. mesh and are obtained directly from a conversionprocess. However, the diamond particles utilized can be obtained fromlarger sized materials which are milled or pulverized by conventionaltechniques.

The diamond abrasive particles are initially coated with acarbide-forming metal such as silicon, chromium, titanium, tungsten,zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, or acarbide, boride, or nitride thereof. Chromium is preferred. A suitablemethod for depositing some of these metals, such as chromium, is apacked salt cementation process operating between about 600°-1000° C.for chromium, preferably between 800°-950° C. Abrasive diamond particlesare typically immersed within a molten bath of one or more alkali oralkaline earth halides with the carbide-forming metal. This techniqueallows for chemical bonding of the carbide-forming metal to the diamondparticle surface on formation of metal carbide. The details of asuitable salt bath deposition process can be found in U.S. Pat. No.2,746,888. The carbide-forming layer can be applied in a wide range ofthicknesses. Chromium is preferably at a thickness ranging from 0.1 to10 μm, more preferably 2-5 μm.

Other methods for applying the primary carbide-forming layer are alsosuitable if significant chemical bonding is obtained with limitedexposure to harmful thermal cycles. Chemical vapor deposition (CVD)techniques are most preferred and, more preferably, low pressurechemical vapor deposition (LPCVD) techniques are used. LPCVD techniquesare well known in the art. These techniques utilize reactive gasmixtures at subatmospheric conditions and high substrate temperatures todeposit carbide-forming metals, such as chromium. Prior to coating thediamond abrasive particles, it is preferable to remove oxides andvolatile impurities from the surface, particularly surface oxidecontaminants. A suitable technique for removing these impurities anddepositing metal layers by LPCVD is described in U.S. Pat. No.4,289,503, which is directed to removing oxides on cubic-boron nitride.

The secondary layer is deposited by electroless deposition to providetools with improved wear performance. Preferably, these secondary layersare thicker than the primary layer and provide a rough surface.

Non-carbide forming metals are preferred for use as the secondary layersuch as nickel, cobalt, and iron. Of these metals, nickel and cobalt arepreferred. Preferably, the electroless deposition is preformed from ametal hypophosphite solution. A nickel/hypophosphite solution depositsboth nickel and a small percentage of phosphorus (6-11 wt %). A suitableelectroless deposition process is described in U.S. Pat. No. 3,556,839.

While the coated abrasive particles of the present invention typicallycomprise only one primary carbide-forming layer and onenon-carbide-forming secondary layer, additional layers ofnon-carbide-forming metals are optional. For example, thinnickel/phosphorus layers applied by electroless deposition techniquesbetween the chromium layer and outer coatings of cobalt or iron can beused.

The secondary metal coating is preferably applied at a level of about10-50 wt % of the abrasive particles. Most preferably, the coating isapplied in an amount of between 20-35 wt % of the abrasive particles.The primary coating is relatively thin; so the total coating applied mayrange from above 10 wt % to about 60 wt % of the diamond abrasive.Preferred levels for the total coating fall within the range of 20-40 wt% of the uncoated particles. The thickness of the metal coating may bevaried to control properties of tools such as particle retention,lubrication and heat diffusion characteristics. One of ordinary skill inthe art can vary the coating thicknesses and diamond granule sizes forthe tool intended by routine investigation. After the diamond abrasiveparticles are coated with multiple metal layers, they are used to forman abrasive tool bonded by a sintered metal.

The coated diamond abrasive particles are impregnated within a suitablemetal matrix by conventional techniques when used in abrasive tools. Forexample, a mixture of the coated abrasives and metal particles can bepressed at ambient temperature to the shape desired and the pressedarticle heated so as to sinter the metal therein. Suitable metalsinclude nickel, cobalt, etc. Preferred components are tool inserts forsaw blades of 30-40 mesh size diamond particles coated with chromium andnickel and bound by a sintered nickel, cobalt, and/or cobalt/bronzematrix. These tool inserts can be of any form or shape, particularlythose shapes which are conventional for tools used to cut stone andconcrete.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing and in the following examples, all temperatures are setforth in degrees Celsius and unless otherwise indicated, all parts andpercentages are by weight.

The entire disclosure of all applications, patents and publications,cited above and below, are hereby incorporated by reference.

EXAMPLES Chromium-Coated Diamond Abrasives

Synthetic diamond abrasive particles under the trade designationsMBS-70, MBS-750, and MBS-760, all of 30/40 mesh size, provided by theGeneral Electric Company, are each separately coated with chromium in asalt bath. The salt bath comprises chromium metal and a mixture of saltsincluding sodium chloride, potassium chloride, and calcium chloride. Thetemperature of the bath is maintained at between 850°-900° C. Afterabout two hours of treatment, the particles have a chromium coating offrom 0.5-1.0 μm in thickness.

Titanium-Coated Diamond Abrasives

Synthetic diamond abrasive particles under the trade designationsMBS-70, MBS-750, and MBS-760, all of 30/40 mesh size, provided by theGeneral Electric Company, are each separately coated with titanium in asalt bath. The salt bath comprises titanium metal, NaCl, KCl, and CaCl₂.The temperature of the bath is between 850-900/C. After about two hours,the particles have a titanium layer of about 0.25-1.0 μm in thickness.

Secondary Coating: Electroless Deposition

Chromium-coated MBS-750 diamond abrasive particles and titanium-coatedMBS-750 diamond abrasive particles, as described above, are separatelyover coated with a cobalt/phosphorus layer by an electroless depositionprocess consistent with the procedures described in U.S. Pat. No.3,556,839. Chromium-coated MBS-760 and titanium-coated MBS-760 areseparately over coated with a nickel/phosphorus layer. A hypophosphitesolution of cobalt or nickel is used as the plating solution. The pH isbetween 4-5.5 for the nickel solution and 12-14 for the cobalt solution.The process temperature maintained at between 60°-95° C. The metallayers are individually deposited from a series of separate baths. Whenthe nickel or cobalt is depleted, the bath is discarded, and a freshbath is used until the desired coating weight is obtained. Typically,from 5-20 baths are used, depending on the size of the bath container,the size of the abrasive particles, and the initial concentration of thebath. The nickel/phosphorus layers and the cobalt/phosphorus layers aredeposited in an amount of from 20-40 wt %, based on the original weightof the abrasive particles, and have a phosphorus content of from 6-11 wt%.

Secondary Coating: Electrolytic Deposition

Portions of the titanium-coated and chromium-coated MBS-70, MBS-750, andMBS-760 diamond abrasives described above are separately over coatedwith either a nickel or cobalt layer by a conventional electrolyticdeposition technique. The amount of nickel or cobalt applied ranges from20-40 wt %, based on the original weight of the abrasive particles.

Particle Retention

Uncoated and coated diamond abrasive particles (MBS-70, MBS-750, andMBS760), as described above, are separately bound within test bars usinga conventional cobalt matrix for stone-cutting abrasive tools.

The test bars are obtained by mixing the abrasive particles describedabove with the matrix alloy in powder form in a ratio conventionallyused for abrasive tools. The mixture is compressed into the shape of atest bar, sintered at conventional temperatures used for the matrixalloy, and cooled to ambient temperature. The test bars are 2" long,1/4" wide, and 1/4" in height.

The relative diamond retention in the bars produced is tested bybreaking the bar across its width in an Instrom machine, which applies aconstant force (about 2.0×10⁴ m/min.). The percent retention reported inTables I-III below is equivalent to the percentage of broken diamondcrystals across the break of the test bar.

                  TABLE I                                                         ______________________________________                                        Abrasive particle MBS-70                                                                         Retention                                                  ______________________________________                                        Uncoated           10%                                                        Cr                 20%                                                        Cr/Co              33%                                                        Cr/Ni              44%                                                        Ti                  6%                                                        Ti/Ni              52%                                                        ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        Abrasive Particle MBS-750                                                                         Retention                                                 ______________________________________                                        Uncoated             4%                                                       Cr                  17%                                                       Cr/Ni               55%                                                       Cr/Co               56%                                                       Cr/Co--P (electroless)                                                                            32%                                                       Ti                   8%                                                       Ti/Ni               26%                                                       Ti/Co--P (electroless)                                                                            22%                                                       ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        Abrasive Particle MBS-760                                                                         Retention                                                 ______________________________________                                        Uncoated             2%                                                       Cr                  28%                                                       Cr/Ni               47%                                                       Cr/Co               58%                                                       Cr/Ni--P (electroless)                                                                            62%                                                       Ti                   5%                                                       Ti/Ni               42%                                                       Ti/Ni--P (electroless)                                                                             5%                                                       ______________________________________                                    

These results are approximate but show that the coated diamond particlesof the invention are significantly superior in retention to uncoateddiamond abrasive particles and diamond particles coated with only onelayer (chromium or titanium). In addition, this data shows thatparticles with a multi-layer coating having a primary layer of chromiumare consistently superior in retention to particles with a titaniumprimary layer.

Wear Performance

Synthetic diamond abrasive particles under the trade designationsMBS-750 and MBS-760, both of 30/40 mesh size, provided by the GeneralElectric Company, are each coated by the procedures described above infive variations. These include a mono-layer of carbide-forming chromium,a dual layer of carbide-forming chromium, and either an electrolessnickel or cobalt layer and a dual layer of carbide-forming chromium andeither an electroplated cobalt or nickel layer. The composition andquantity (wt %) of metal coating on the different particles, numbered1-10, are shown below. The total weight percentages of the combinedmetal layer coatings on the then different coated particles are asfollows:

    ______________________________________                                                        MBS-750/Particle No.                                          Cr-inner         6.03/1                                                       Cr/Co electroplate                                                                             27.1/2                                                       Cr/Co--P electroless                                                                          23.95/3                                                       Cr/Ni electroplate                                                                            27.13/4                                                       Cr/Ni--P electroless                                                                          25.33/5                                                                       MBS-760/Particle No.                                          Cr-inner         4.07/6                                                       Cr/Co electroplate                                                                             24.6/7                                                       Cr/Co--P electroless                                                                          27.89/8                                                       Cr/Ni electroplate                                                                            24.62/9                                                       Cr/Ni--P electroless                                                                           24.8/10                                                      ______________________________________                                    

The metal coated diamond particles are processed into segments for 7"diameter saw blades. The bond matrix is 100% cobalt, hot-processed under5000 psi at 850/ for three minutes. A total of nine arc segments(0.240"×0.140"×0.20") are induction-brazed onto a circular steel core toproduce 7" nominal diameter blades for testing wear performance.

Prior to testing, each saw blade is conditioned by blade open by sawinga sandstone block and sawing Barre granite under testing conditions todevelop a stabilized cutting surface. The blade is then measured atthree pre-selected locations long each segment's length (leading edge,center, and trailing edge), and an average of the radial height iscalculated for each saw blade. The radial measurements are made to thenearest 0.0001".

The saw tests are conducted on a modified surface grinder sawing Barregranite at the rate of 46.5 in² /min (300 cm² /min). The sawing iscarried out under alternating upcut and downcut sawing at a 0.344" (10.0mm) depth of cut and a traverse rate of 118.1 in/min (3 m/min). Therotational speed of the blade during all tests was fixed at 5904 SFPM(30 m/sec). Water coolant is applied to the blade during sawing at adelivery rate of 3.5 gal/min (38 l/min).

The results of the 7" blade tests are expressed in a wear performancenumber. This number is calculated by dividing the amount of granitesawed (square inches) by the average blade radial wear in 0.001 inches.Under normal circumstances, the amount of granite sawed by the blades ofany particular test series is determined as the minimal amount necessaryto generate at least 0.010 inches of radial blade wear. The specificwear performance results are shown in Tables IV-VII.

                  TABLE IV                                                        ______________________________________                                        Wear Performance of MBS-750 Diamond                                           Coated With Chromium/Cobalt                                                                                    Average                                      Particle No.                                                                              Coating   Tests 1 and 2                                                                            Wear Life                                    ______________________________________                                        Control Control                                                                           None None                                                                                ##STR1##   164.3                                       2 2         Cr/Co Cr/Co                                                                              ##STR2##   194.8                                       3 3         Cr/CoP Cr/CoP                                                                            ##STR3##   424.8                                       ______________________________________                                    

                  TABLE V                                                         ______________________________________                                        Wear Performance of MBS-750 Diamond                                           Coated with Chromium and Chromium/Nickel                                                                       Average                                      Particle No.                                                                              Coating   Tests 1 and 2                                                                            Wear Life                                    ______________________________________                                        Control Control                                                                           None None                                                                                ##STR4##   103.5                                       1 1         Cr Cr                                                                                    ##STR5##   214.0                                       4 4         Cr/Ni Cr/Ni                                                                              ##STR6##   139.5                                       5 5         Cr/NiP Cr/NiP                                                                            ##STR7##   143.0                                       ______________________________________                                    

                  TABLE VI                                                        ______________________________________                                        Wear Performance of MBS-760 Diamond                                           Coated with Chromium and Chromium/Cobalt                                                                       Average                                      Particle No.                                                                              Coating   Tests 1 and 2                                                                            Wear Life                                    ______________________________________                                        Control Control                                                                           None None                                                                                ##STR8##   186.3                                       6 6         Cr Cr                                                                                    ##STR9##   178.5                                       7 7         Cr/Co Cr/Co                                                                              ##STR10##  155.3                                       8 8         Cr/CoP Cr/CoP                                                                            ##STR11##  209.3                                       ______________________________________                                    

                  TABLE VII                                                       ______________________________________                                        Wear Performance of MBS-760 Diamond                                           Coated with Chromium and Chromium/Nickel                                                                       Average                                      Particle No.                                                                              Coating   Tests 1 and 2                                                                            Wear Life                                    ______________________________________                                        Control Control                                                                           None None                                                                                ##STR12##  168.0                                        6  6       Cr Cr                                                                                    ##STR13##  163.8                                        9  9       Cr/Ni Cr/Ni                                                                              ##STR14##  160.5                                       10 10       Cr/NiP Cr/NiP                                                                            ##STR15##  203.5                                       ______________________________________                                    

The above data shows that particles with multi-layer coatings having anouter secondary metal coating deposited electrolessly provide superiorwear performance over particles with secondary metal coatings depositedelectrolytically and also uncoated particles. Some of the multi-layercoatings with a secondary layer of electrolytically deposited cobaltprovide superior wear performance over similar particles with only achromium mono-layer.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. An abrasive particle suitable for use in sinteredmetal bonded abrasive tools comprising a diamond abrasive having amulti-layer coating consisting essentially of one homogeneous carbideforming metal primary layer chemically bonded to the surface of thediamond abrasive and at least one non-carbide forming metal secondarylayer deposited electrolessly.
 2. An abrasive particle as in claim 1,wherein said non-carbide forming metal secondary layer comprisescobalt/phosphorus, iron/phosphorus, or nickel/phosphorus.
 3. An abrasiveparticle as in claim 2, wherein the amount of said non-carbide formingmetal secondary layer ranges from 10 to 50 wt %, based on the weight ofthe uncoated diamond abrasive particle.
 4. An abrasive particle as inclaim 2, having two different non-carbide forming metal secondary layersdeposited electrolessly.
 5. An abrasive particle as in claim 1, whereinthe diamond abrasive particles have an average particle size in therange of 1 to 1500 μm.
 6. An abrasive particle suitable for use in asintered metal bonded abrasive tool comprising a diamond abrasive havinga multi-layer coating consisting essentially of one homogeneous carbideforming metal primary layer deposited by chemical vapor deposition, orpacked salt deposition and a non-carbide forming metal secondary layerdeposited by electroless deposition.
 7. An abrasive particle as in claim6, wherein said non-carbide forming metal secondary layer comprisescobalt/phosphorus or nickel/phosphorus.
 8. An abrasive particle as inclaim 6, wherein the carbide forming metal primary layer has a thicknessranging from 0.1-10 μm.
 9. An abrasive particle suitable for use insintered metal bonded abrasive tools comprising a diamond abrasivehaving a multi-layer coating consisting essentially of one homogeneouschromium metal primary layer chemically bonded to the surface of thediamond abrasive and at least one non-carbide forming metal secondarylayer deposited electrolessly.
 10. An abrasive particle as in claim 9,wherein the amount of said non-carbide forming metal secondary layerranges from 10 to 50 wt %, based on the weight of the uncoated diamondabrasive particle.
 11. An abrasive particle as in claim 9, wherein thenon-carbide forming metal secondary layer is comprised of one layer ofnickel/phosphorus or cobalt/phosphorus.
 12. An abrasive particle as inclaim 9, having two different non-carbide forming metal secondary layersdeposited electrolessly.
 13. An abrasive particle as in claim 9, whereinthe diamond abrasive particles have an average particle size in therange of 1 to 1500 μm.
 14. An abrasive particle suitable for use in asintered metal bonded abrasive tool comprising a diamond abrasive havinga multi-layer coating consisting essentially of one homogeneous chromiummetal primary layer deposited by chemical vapor deposition, or packedsalt deposition and a non-carbide forming metal secondary layerdeposited by electroless deposition.
 15. An abrasive particle as inclaim 14, wherein said non-carbide forming metal secondary layercomprises cobalt/phosphorus or nickel/phosphorus.
 16. An abrasiveparticle as in claim 14, wherein the chromium metal primary layer has athickness ranging from 0.1-10 μm.
 17. A process for the preparation ofcoated diamond abrasive particles for use in sintered metal bondedabrasive tools, said process comprising applying one homogeneous carbideforming metal primary layer to the surface of said diamond abrasiveparticles by chemical vapor deposition or packed salt deposition andapplying at least one secondary layer of a non-carbide forming metal byelectroless deposition.
 18. A process according to claim 17, wherein thehomogenous carbide forming metal primary layer consists essentially ofchromium and is chemically bonded to the diamond abrasive particlewithout a separate sintering step.
 19. A process according to claim 17,wherein the carbide forming metal primary layer is applied at athickness ranging from 0.1 to 10 μm, and the secondary layer is appliedin an amount in the range of 10-50 wt %, based on the weight of theuncoated diamond abrasive particles.
 20. A process according to claim17, wherein two different secondary layers are applied by electrolessdeposition.
 21. A method according to claim 17, wherein the secondarylayer is deposited from at least one of nickel hypophosphite or cobalthypophosphite solution.
 22. A process according to claim 17, comprisingthe additional steps of mixing the coated diamond abrasive particleswith a sinterable metal, pressing the mixture at ambient temperature toform a solid mass of a desired shape and heating the solid mass to atemperature sufficiently high to sinter said sinterable metal.
 23. Asintered metal bonded abrasive tool comprising a sintered metal matrixand abrasive particles of claim
 1. 24. An abrasive tool which comprisesa sintered metal matrix and multi-layer coated diamond abrasiveparticles wherein the multi-layer coating consists essentially of onehomogenous carbide-forming primary metal layer chemically bonded to thesurface of the diamond abrasive and at least one non-carbide formingmetal secondary layer deposited electrolessly.
 25. An abrasive toolaccording to claim 24, wherein the primary carbide forming metal primarylayer is applied by packed salt deposition or chemical vapor depositionand the non-carbide forming metal secondary layer is applied byelectroless deposition, and said carbide forming metal primary layerconsists essentially of chromium, and said non-carbide forming metalsecondary layer comprises cobalt/phosphorus or nickel/phosphorus.
 26. Anabrasive tool according to claim 24, which is an insert for a saw blade.